1. Field of the Invention
This invention generally relates to ring lasers, and more specifically ring laser angular rate sensors. Yet more specifically, the invention relates to the selection of the transverse mode of the counter-propagating laser beams which propagate along the optical closed loop path of the ring laser in order to selectively diminish the inherent bias stability associated with the ring laser angular rate sensor performance.
2. Description of the Related Art
Ring laser angular rate sensors or ring laser gyros of the type manufactured by Honeywell, Inc., of Minneapolis, Minn. are well known. As its name implies, a ring laser gyro is an angular rate sensor which utilizes a laser beam traveling in a closed loop optical path. More particularly the Honeywell ring laser gyro is an angular rate sensor which utilizes a pair of laser beams which are directed by a plurality of reflectors or mirrors to travel in opposite directions along a closed loop optical path within a ring laser block and to detect rotation about an axis passing through the polygonal path circumscribed by the laser beam by detecting differences in the travel characteristics of the two beams.
One type of polygonal closed path employed for ring laser angular rate sensors is triangular, and in which one mirror is positioned or located at each corner of the triangular path. Other types of ring laser rate sensors or gyros having other polygonal shaped optical closed loop paths, such as four sided ring laser gyros, are also known, and they operate according to the same principles.
As is also well known, in order to properly operate a ring laser gyro, the ring laser gyro requires that the closed loop polygonal optical path be maintained at a substantially constant length. This is important since the laser beam intensity is dependent upon the closed loop polygonal optical path, hereinafter referred to as the optical path length. Variations in the beam intensity can adversely affect the performance parameters of the gyro and such variations can cause gyro errors. In order to maintain a constant optical path length, a mirror transducer is commonly employed in which one of the mirrors in the closed loop optical path includes a mechanical transducing means for adjusting its position relative to the other mirrors which also define the optical closed loop path.
Such mirror transducers compensate the ring laser gyro for thermal expansion effects which are inherent in the structure, and which may cause undesirable optical path length variations.
The operation of a ring laser gyro with employment of a path length control mirror transducer is well known. Mirror transducers for path length control in ring laser gyros have generally been fabricated by use of a piezo electric element driven transducer assembly. Examples of path length control mirrored transducers used in ring laser gyro applications include those illustrated in U.S. Pat. No. 3,581,227 and U.S. Pat. No. 5,420,685, both of which were issued to Podgorski, U.S. Pat. No. 4,383,763 issued to Hutchings, et at., U.S. Pat. No. 4,488,080 issued to Baumann, and U.S. Pat. No. 4,691,323 issued to Ljung, et al.
Path length control systems are also shown and described in U.S. Pat. No. 4,152,071, issued to Podgorski, and U.S. Pat. No. 4,522,496 issued to Sanders. Each of these patents show in further detail the employment of a partially transmissive mirror for permitting a portion of the laser beam to exit and impinge upon a photodetector. In turn, the photodetector provides an output signal representative of the intensity of the beam impinging thereupon. A control scheme employing generally a synchronous demodulator and integrator provides a drive signal to the path length control mirror transducer for adjusting the laser path length to obtain a peak laser beam intensity. As is well understood in the art, the closed loop optical path length traveled by the laser beams may be adjusted by more than one mirror. For example, a pair of mirrors, either in the triangular optical closed loop path or the rectangular optical closed loop path scenario.
Optical arrangements for obtaining a single beam signal representative of the intensity of the individual beams or a double beam signal to determine an output which is proportional to the frequency difference of the counter-traveling laser beams and thus to the angular rotation are known. U.S. Pat. No. 4,677,641 describes an optical readout arrangement for obtaining both the single beam signals and the double beam signals and U.S. Pat. No. 4,514,832 describes the detector structure for use in the ring laser gyro, also for obtaining information representative of the single beam signals and the double beam signals.
Ring laser gyros of the prior art, including the path length control transducers as aforementioned, have been generally constructed such that the pair of laser beams are established and propagate in only the fundamental transverse electromagnetic wave mode identified as TEM.sub.00.
One technique used by the prior art for only permitting the existence of the TEM.sub.00 laser beam mode is by restricting the laser beams by passing them through a limited aperture or tunnel thereby preventing the existence of other transverse modes.
It is recognized by those skilled in the art that TEM.sub.00 modes are to be distinguished from other transverse modes. For example, in the aforementioned U.S. Pat. No. 4,522,496, a ring laser gyro is described in which there exists several transverse modes for the same co-existing laser beams.
Further, other mode controls regarding lasers and masers have been described in U.S. Pat. No. 3,286,193 issued to Koester et at. and U.S. Pat. No. 4,219,254 issued to Macken. Macken describes a technique for controlling the type of transverse electromagnetic modes in a laser by employment of an optical device having an arrangement and type of coatings thereon. A mode control is also illustrated in U.S. Pat. Nos. 4,519,708 issued to Perlmutter, et al. and 4,627,732 issued to Braun, et al. The latter two patents addressing ring laser gyro operation for a four frequency gyro including techniques for operating the gyro with particular left and right circular polarized waves in a specific manner to avoid the frequency locking or locking problem as is well understood with regard to two frequency laser gyroscopes.
In general, discussion of laser modes can also be found in publications relating to lasers. For example, reference is made to Bela A. Lengyel's "Introduction to Laser Physics" John Wiley and Sons, Inc., N.Y, and a publication entitled "Quantum Electronics", by Amnon Yariv, John Wiley and Sons, Inc., New York, London, Sydney, Toronto, copyright 1967, 1975, on page 119, figures and including particularly FIG. 6.7 showing various transverse electromagnetic wave modes. Another publication is entitled "Resonant Modes in a Maser Interferometer" by A. G. Fox, et al., found in the Bell System Technical Journal, March 1961.
U.S. Pat. No. 3,879,130 issued to Greenstein illustrates another example of a ring laser being operated with the counter-propagating laser beams being of two different longitudinal modes, i.e., different frequency.
One attribute of a ring laser gyro for judging performance is bias stability. As is well understood, if bias in a ring laser gyro is stable it may be compensated. On the other hand, if the bias is unstable, then it may only be compensated to a limited extent, and the remainder results in long-term and unknown error. Since the bias stability is a function of the arrangement of all of the assembly of the ring laser gyro, including all its constituent components, it may not be predicted except for extreme values. Therefore, ring laser gyros are commonly subjected to extensive performance tests so as to ascertain the bias stability characteristics of the completed gyro before being placed in service in a navigation or control device.
In turn, if the bias stability is unacceptable, the ring laser gyro will be disassembled and parts may be salvaged in order to produce another gyro for being reassembled and performance testing. Of course, such procedures directly contribute to the total cost of the ring laser gyro.
As stated above, although other modes have been recognized as existing, all attempts have been made in previous laser gyros to restrict their operation to the TEM.sub.00 mode.